This paper presents a monotonic multi-switching technique that is implemented in a 8b SAR ADC. The proposed switching reduces 1/2 total DAC capacitance and achieves more than 80% switching energy saving when compared to the most advanced VCM-based or merged capacitor switching methods. Besides, conversion redundancies are added to compensate the errors resulting from insufficient DAC settling and reference noise. The proposed 8-bit SAR ADC operates at 100MS/s with 0.6V supply in 65nm CMOS. The simulation results show that the design achieves 48.8dB SNDR with only 0.524mW power. The Figure-of-Merit (FoM) is 23.35fJ/conversion-step.